The fluidized bed gas phase polymerization technology was developed by Union Carbide in the mid 1970's. In the condensed mode process a gas comprising monomers, a ballast gas, and a condensable liquid is circulated through a bed of reactive catalyst in the reactor, through a disengagement zone at the top of the reactor (i.e. the expanded upper section of the reactor) where entrained polymer particles drop back into the bed, through a recycle line including a compressor, and a condenser where some of the components in the gas phase may be liquefied, and back to the bottom of the reactor. The recycle stream enters the bottom of the reactor through a deflector plate. In theory the stream is evenly distributed by passing through a bed plate and into the reactor. However, the flow pattern below the bed plate is quite complex due to the presence of a jet created by the recycle gas pipe flow. In practice the distribution may be uneven resulting in one section or the bed having more condensed phase than another. In some older reactors the product discharge port may be quite close to the bed plate. If there is an uneven distribution of liquids within the bed the discharge port may be flooded with liquid. Also an uneven distribution of liquids within the bed may result in “mud” and potentially sheeting.
U.S. Pat. Nos. 4,543,399 and 4,588,790 issued to Jenkins, III et al. Sep. 24, 1985 and May 13, 1986, respectively, assigned to Union Carbide Corporation teach incorporating into the feed stream up to about 20 weight % of the recycle stream of a condensable gas i.e., a gas which condenses or at least partially condenses to liquid when passed through a heat exchanger prior to being recycled back to the reactor sometimes also referred to as an Induced Condensing Agent). Typically these gases are C4-6 alkanes, preferably isomers of pentane and hexane.
U.S. Pat. Nos. 5,462,999 and 5,436,304 to Griffin et al. issued Oct. 31, 1995 and Jul. 25, 1995, respectively, and U.S. Pat. Nos. 5,405,922 and 5,352,749 to DeChellis et al. issued Apr. 11, 1995 and Oct. 4, 1994, respectively, all assigned to Exxon Chemical Patents, Inc. all teach operating a gas phase polymerization where in the feed stream may contain from about 17.5 up to 50 weight % of a condensable gas.
There are a number of United States patents in the name of Rhee et al. assigned to Union Carbide Corporation. These patents disclose deflector plates which may be annular (see FIG. 2 A of U.S. Pat. No. 4,933,149) for use in fluidized bed gas phase reactors operated in condensing mode. The deflector plate is in the bottom of the reactor above and proximate to the reactor inlet. From the example the distributor plate or bed plate is 2.54−0.10=2.44 meters (about 7.9 feet) above the upper surface of the deflector plate. The volume of the reactor below the bed plate is referred to as the mixing chamber (Col. 3 lines 40-45). The annular deflector plate divides the incoming gas stream into a central upward flow and a flow directed to the side walls to “sweep” the wall of the mixing chamber (Col. 4 lines 1-5) reduce filming on the reactor wall in the mixing zone and also to re-entrain any liquids which have separated from the gas stream (Col. 9 line 65-Col. 10 line 10). Below the bed plate is a screen 27 to reduce the likelihood of the bed plate becoming plugged by resin chips impinging on the plate as the recycle stream carries chips upwards (Col. 9 line 52; Col. 10 lines 60-66; and Col. 11 lines 39-43).
Interestingly in FIG. 2 there appear to be some un-described elements beneath the bed plate. This structure is not described anywhere in the patent application.
U.S. Pat. No. 6,723,291 issued Apr. 20, 2004 to Wu, assigned to China Petrochemical Corporation and Tianjin United Chemicals Corporation, disclosed a deflector plate wherein the annular opening of the deflector plate of U.S. Pat. No. 4,933,149 has an inverted truncated open cone placed over it. This divides the incoming gas into three segments. This improves the flow of gases in the mixing chamber overcoming some disadvantages of the distributor plate of Rhee. The distributor plate can be used in condensed more of operation containing up to 25 wt. % of liquids.
Interestingly the screen and the unidentified structures dependent from the bed plate have been eliminated.
U.S. Pat. No. 7,446,156 issued Nov. 4, 2008 to Dooley assigned to Westlake describes without drawings by reference to a provisional application “swirling” the bed of particles in the reactor to reduce sheeting.
While the figures in the prior art show a uniform circulating flow pattern through the mixing zone the flow may in fact be non uniform in three dimensions. This is particularly the case if the central upward gas flow of the mixing chamber impinging on the bottom of the bed plate sees a “raceway” or an unimpeded direction of flow. The result is that the raceway preferentially directs the flow in some areas beneath the bed plate and not to other areas causing secondary or tertiary flow patterns. The flow along a raceway across the bottom of the bed plate extends to the outer edge of the bed plate, where it redirects and collides with a portion of upward flow from the distributor plate resulting in an increased flow of gas and liquids in that peripheral area of the bed plate. This results in liquid flooding of the bed of polymer particles above the area of higher concentration of liquid droplets. The situation is exacerbated in some reactor designs where the outlet for the polymer is less than roughly half a diameter of the bed plate above the bed plate. This may result in the product discharge system becoming flooded with condensed liquids.
The present invention seeks to improve product quality (e.g. polydispersity, CDBI, etc.), and reduce reactor bed flooding, product discharge flooding and sheet formation by eliminating raceways on the bottom surface of the bed plate.